1. Field of the Invention
The present invention relates to a fuel injection control device of a diesel engine equipped with an exhaust gas after treatment device.
2. Description of the Related Art
In diesel engines, it is very important to reduce the contents of particulate matter (PM), nitrogen oxides (NOx) and HC (hydrocarbon) in exhaust gas, and a variety of after treatment devices for reducing those contents have been suggested.
For example, a DPF (diesel particulate filter) of a continuous regeneration type comprises an oxidation catalyst for oxidizing the HC present in exhaust gas and a catalyst-equipped filter disposed downstream of the oxidation catalyst and serving to collect, oxidize and remove the PM present in the exhaust gas. In such a DPF of a continuous regeneration type, first, the HC present in the exhaust gas are oxidized and removed with the oxidation catalyst and then the PM present in the exhaust gas is collected with the catalyst-equipped filter. Under the effect of the catalyst supported in the filter, the collected PM is oxidized and removed and the filter is self-regenerated.
However, in after treatment devices using a catalyst action, such as DPF of a continuous regeneration type, a sufficient exhaust gas purification effect cannot be obtained if the temperature of exhaust gas is below the catalyst activity temperature (for example, no less than 250° C.). For example, when the exhaust gas temperature is low, for example, immediately after the engine is started or in a low-load driving mode, the PM collected by the catalyst-equipped filter cannot be oxidized and removed and there is a risk of the filter becoming clogged.
Accordingly, the exhaust gas temperature was raised by executing post-injection (fuel injection for a catalyst) after the compression dead top center, separately from the usual fuel injection, when the exhaust gas temperature has not reached the activity temperature of the catalyst in the after treatment device. Because with such a post-injection, a large amount of non-combusted gas (HC) is discharged, most of the HC are oxidized with the oxidation catalyst and the exhaust gas temperature rises due to the oxidation reaction heat.
When the aforesaid post-injection is executed, the injection quantity of post-injection is typically determined based on the difference between the exhaust gas temperature which is considered as a target (catalyst activity temperature) and the actual exhaust gas temperature. Therefore, a comparatively large quantity of fuel is post injected when the difference between the target exhaust gas temperature and the actual exhaust gas temperature is comparatively high, for example, immediately after the execution of the exhaust gas temperature increase control by post-injection has been started.
However, when the difference between the target exhaust gas temperature and the actual exhaust gas temperature is high, the temperature of the oxidation catalyst is low and, therefore, the HC treatment capacity of the catalyst is low. For this reason, the problem was that when a large quantity of fuel was post injected, HC were discharged in an amount exceeding the treatment capacity of oxidation catalyst and the HC that could not be treated were discharged as a white smoke.